In the metal forming art there are numerous ways of forming cup-shaped articles. One method includes pouring a molten metal into a mold to form a cup-like shell. Another method includes forming a cylinder of sheet metal, and welding a cap to one end thereof. Yet another method includes drawing the metal from a planar sheet of metal to form the cup-shaped article. While the noted methods are all effective in forming the final article, the latter method is the most expedient, as it is less costly and requires much less time. A method of forming a cup-shaped article by the latter noted drawing process is disclosed in U.S. Pat. No. 4,509,356, and assigned to the assignee hereof.
Among other applications, cup-shaped articles are utilized in constructing fragmentation containers, such as employed in manufacturing military grenades. In this application, the inside surface of the cup-shaped article is embossed to form lines of weakness and thereby facilitate the fragmentation of the article when exploded.
Embossing a grid pattern of grooves by techniques heretofore known in the art includes the forming of the pattern while the material is flat, in either a blank or strip form. The preferred practice is to roll form the material by passing it through a pair of embossing rollers, and then rewinding the strip in a coil for subsequent processing. One of the embossing rollers is machined to impress the emboss design or pattern on one side of the metal strip, while the other roller is a smooth surface used to compress the metal strip against the embossing roller. When this technique is employed, alloy steels which form the blank or strip material tend to work-harden. Work-hardening of the material is alleviated by annealing the strip material after the roll forming of the emboss design. The annealing operation is expensive, as it is necessary to heat the embossed material to a temperature in excess of 1,000.degree. F., under a controlled atmospheric condition. The embossed strip is then formed into the cup-shaped article by one of the methods identified above.
Other techniques for forming a grooved structure on the inside surface of a bore or cylinder concerns an extrusion process which is adapted for forming rifling grooves within a tubular structure. Such a process is briefly described in the article "Extrusion of Thin-Wall Tubes Eliminates Machining", by William G. McEwen, P. 89-90, The Tool and Manufacturing Engineer, December 1963. According to this process, a steel billet has a bore formed therethrough, and a mandrel, ram and ring die are utilized to extrude the billet over the mandrel and thereby form the riflings. This process is not only expensive and time consuming, but also can be accomplished only on open-ended cylindrical articles. One end of the extruded cylinder must then be welded or otherwise capped to form the cup-shaped article.
It can be seen that a need exists for a high-speed and economical method and apparatus for forming an embossed design on the inside surface of cup-shaped articles. A concomitant need exists for combining embossing and drawing operations of the material to facilitate the construction of the embossed article. An additional need exists for a high-speed drawing and embossing operation so that the forming is completed before work hardening occurs. The expensive and time-consuming annealing process can then be eliminated.